Failure to parse f-strings when field has `=` at the end

[Ark-kun]

Fails when f-string field has = at the end:

f"aaaa {sys.version_info=} bbb"

  File "pasta/base/annotate.py", line 1537, in visit_FormattedValue
    self.attr(
  File "pasta/base/annotate.py", line 1864, in attr
    attr_parts.append(self.token(attr_val))
                      ^^^^^^^^^^^^^^^^^^^^
  File "pasta/base/annotate.py", line 1774, in token
    raise AnnotationError(
pasta.base.annotate.AnnotationError: Expected '!' but found '='
line 1: sys_version=} is inconsistent with {sys.version_info=}. "

https://docs.python.org/3/reference/lexical_analysis.html#formatted-string-literals

When the equal sign '=' is provided, the output will have the expression text, the '=' and the evaluated value. Spaces after the opening brace '{', within the expression and after the '=' are all retained in the output. By default, the '=' causes the repr() of the expression to be provided, unless there is a format specified. When a format is specified it defaults to the str() of the expression unless a conversion '!r' is declared.

Added in version 3.8: The equal sign '='.